1. Field of the Invention
The invention relates to a heat sink of the type having base plate and a heat pipe with a flat surface which is brought into contact with a device to be cooled, such as a central processing unit (CPU).
2. Description of the Related Art
Heat sinks utilizing heat pipes are well known. A heat pipe generally consists of a tube forming a closed volume containing a heat transfer fluid which is present in two phases. The tube is preferably lined with a wicking material which distributes the liquid phase within the closed volume, and in particular draws it from a condenser section back toward an evaporator section. The condenser section is generally in contact with cooling fins or other means for removing heat, while the evaporator section is in contact with the device to be cooled.
U.S. Pat. No. 7,059,391 discloses a heat sink utilizing a base plate having a pair of slots in which the ends of a heat pipe are received to form a evaporator sections which are mounted on a CPU. The exposed portions of the heat pipe on the bottom surface of the plate may be machined to present a flat surface to the CPU. The condenser section is formed by a loop of the heat pipe which passes over a wall on the top side of the heat sink and is flanked by cooling fins extending parallel to the plate. This is a relatively high profile design which is not suitable for applications where space above the mounting surface is limited.
U.S. Pat. No. 7,117,930 in FIG. 7 discloses a heat sink with a base plate having a bottom surface in which a central portion of a heat pipe is press fit so that it forms an evaporator section which is flush with the bottom surface. Here too the exposed portions of the heat pipe may be machined so as to be flat and smooth. The condenser section of the heat pipe is formed by ends of the heat pipe which extend upward from the top surface through cooling fins which are parallel to the plate. Since the base plate is designed to be extruded, the long sections of heat pipe which form the evaporator section cover a large area, which does not cool a highly concentrated heat source such as a CPU with great efficiency.
US 2007/0074857 discloses a heat sink including a base plate having a top surface provided with grooves, and an opposed bottom surface which is installed against a CPU. Multiple heat pipes, in particular two pairs of U-shaped heat pipes, are installed in the grooves so that one arm of each heat pipe is juxtaposed against respective arms of other heat pipes to form evaporator sections directly opposite from the area of the bottom surface which contacts the CPU. The heat pipes are coplanar with the top surface, which is provided with cooling fins.
FIG. 9 illustrates another heat pipe arrangement according to the prior art. Here an open channel in the surface of a plate accommodates a pair of U-shaped heat pipes, wherein each arm of each heat pipe is juxtaposed against a respective arm of the other heat pipe. The entire arrangement is recessed below the surface of the plate, which is intended for mounting against a heat sink. The object or objects to be cooled, such as a CPU, are mounted against the opposite surface without regard to the position of the heat pipes. As such, no particular sections of the heat pipes serve as evaporator sections or condenser sections; the device is intended to be used as a heat spreader.
In general, heat sinks utilizing heat pipes are limited in their heat removal ability, because the fluid has only one path returning to the evaporator along the length of the pipe, and the heat source is only partially covered by the evaporator section. Vapor chambers can spread the heat generated by high power components over a large area of the base plate, but are relatively expensive, less robust structurally, and difficult to seal. An example of a vapor chamber is disclosed in U.S. Pat. No. 7,306,027.
While heat sinks having heat pipes with evaporator sections covering the heat sink are known (US 2007/0074857), the amount of metal interposed between the vaporizing fluid and the object to be cooled offers higher than optimal thermal resistance and therefore worse performance.
The prior art points to a need for a heat sink having the heat removal advantages of a vapor chamber, but the structural strength and lower manufacturing cost of a heat pipe design.